Cube corner cavity based retroreflectors with transparent fill material

ABSTRACT

Retroreflective sheeting includes a body layer having a structured surface with recessed faces forming cube corner cavities. A reflective film is disposed at least on the recessed faces, and a fill material fills the cube corner cavities. The fill material comprises radiation-curable materials, adhesives, or both, and preferably transparent radiation-curable pressure-sensitive adhesives. The fill material preferably forms a continuous layer covering both the recessed faces and upper portions of the structured surface. A transparent cover layer preferably contacts the fill material layer.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a division of application Ser. No.09/228,367, filed Jan. 11, 1999. The present invention relates to U.S.application Ser. No. 09/227,963, “Cube Corner Cavity BasedRetroreflectors and Methods For Making Same”, filed on Jan. 11, 1999 andincorporated by reference.

BACKGROUND

The present invention relates generally to retroreflective articles suchas sheeting. More particularly, the invention relates to such articlesor sheetings in which retroreflective elements comprise reflective facesarranged to form a cavity.

The reader is directed to the glossary at the end of the specificationfor guidance on the meaning of certain terms used herein.

Cube corner retroreflective sheetings can generally be categorized asthose that use a rear-surface body layer and those that use afront-surface body layer. Commercially available cube cornerretroreflective sheetings are of the former type, in which a thintransparent body layer has a substantially planar front surface and arear structured surface comprising a plurality of geometric structuresof pyramidal shape, some or all of which include three reflective facesconfigured as a cube corner element. Light is incident on the planarfront surface, passes through the thickness of the body layer, and isretroreflected by the cube corner elements back through the frontsurface. In some known embodiments, a reflective coating such asaluminum is applied to the rear structured surface, followed by anadhesive layer that covers and conforms to some extent to the shape ofthe structured surface. However, in general no reflective coating isrequired so long as a clean air interface can be maintained at thestructured surface, in which case reflections occur by total internalreflection.

Some known cube corner retroreflective sheeting constructions use afront-surface body layer, in which the body layer has a front structuredsurface. See, e.g., U.S. Pat. No. 3,712,706 (Stamm), U.S. Pat. No.4,127,693 (Lemelson), and U.S. Pat. No. 4,656,072 (Coburn, Jr. et al.),and PCT Publication WO 89/06811 (Johnson et al.). The front structuredsurface comprises a plurality of reflective faces arranged to form cubecorner cavities. For this reason such retroreflective sheeting isreferred to herein as cube corner cavity based retroreflective sheeting.A thin metal film can be applied to the structured surface to enhancereflectivity of the faces. Incident light does not penetrate through thebody layer but rather is reflected by the faces forming the cube cornercavities. In some embodiments a cover layer that does transmit incidentlight is provided on top of the structured surface to protect thecavities from dirt or other degradation, with portions of the coverlayer extending into and filling in the cube corner cavities of thestructured surface. In other embodiments a cover layer is sealed oradhered to the structured surface by a colored pressure- orheat-sensitive adhesive that cancels, removes, or obliteratesretroreflectivity of the structured surface.

One advantage of cube corner cavity-based retroreflective sheeting isthe ability to use a much wider variety of material compositions for thebody layer than is otherwise possible, since it need not be opticallyclear. Another advantage is the ability to form certain types ofstructured surfaces in the body layer more rapidly than it takes to forma negative copy of such structured surfaces in rear-surface body layerconstructions. This is because molds used to form the structured surfaceof a front-surface body layer can have grooves that are essentiallyunbounded in the direction of the groove. In contrast, molds used toform the structured surface of a rear-surface body layer typically havean array of closed (cube corner) cavities bounded by a plurality ofinverted grooves, i.e., ridges. The unbounded grooves of the formermolds are easier to fill with body layer material than the array ofclosed cavities provided on the latter molds.

Unfortunately, this latter advantage can be essentially nullified inconstructions where the cube corner cavities in the body layer arefilled with a transparent substance. Filling the cavities with such asubstance, referred to as a fill material, is desirable to increase theentrance angularity of the sheeting by refracting highly off-axisincident light closer to the symmetry axis of the cube corner element,as well as to keep dirt or other debris out of the cavities. But suchfilling is undesirable insofar as it requires forcing material into anarray of closed cavities. Such filling is also undesirable to the extentit exposes the body layer to excessive heat, mechanical stress, or otherprocess conditions that could compromise the fidelity of the structuredsurface.

Constructions of the type described would benefit from fill materialshaving properties that make them easy to fill into the cube cornercavities of the body layer, preferably with minimal risk of damaging thefidelity of the structured surface. Preferred fill materials should becompatible with relatively low cost, high flexibility, and highvisibility sheeting constructions.

BRIEF SUMMARY

Certain radiation-curable materials, particularly radiation-curablepressure-sensitive adhesives, have been found to exhibit significantmanufacturing and/or construction advantages when used as fill materialsfor cube corner cavity based retroreflective sheeting.

Retroreflective articles are disclosed having a body layer with astructured surface in which recessed faces define cube corner cavities.A transparent adhesive material fills the cube corner cavities. Theadhesive material is preferably a pressure-sensitive adhesive. In oneembodiment, a release liner covers the fill material. In anotherembodiment, a transparent cover layer takes the place of the releaseliner. The cover layer adds durability to the article, and can alsoincorporate dyes, colorants, or the like to affect the appearance of thesheeting or to convey information.

Methods are disclosed in which a film of reflective material is appliedat least to recessed faces of a body layer structured surface, suchrecessed faces forming cube corner cavities. A flowable composition suchas a resin is applied to the structured surface. The composition is onesuitable for forming a transparent PSA, or one that is radiation curableand suitable for bonding to the film of reflective material, or,preferably, both. After the composition has substantially completelyfilled the cube corner cavities, the composition is crosslinked orotherwise cured by exposure to radiation such as UV light. After theexposure step, the crosslinked composition bonds to the reflective filmand preferably also to a transparent cover layer.

To reduce cost while maintaining functionality and durability, theconstructions preferably utilize thermoplastic materials for the bodylayer and the cover layer. Good flexibility of sheeting articles can beaided by the use of fill materials whose elastic modulus aftercrosslinking is less than about 50,000 psi (345×10⁶ Pascals), andpreferably less than about 25,000 psi (172 MPa).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a retroreflector where a top cover layerthereof is shown only partially laminated to a body layer to reveal cubecorner cavities formed in the body layer;

FIG. 2 is a cross-sectional view of a portion of the retroreflector ofFIG. 1 taken along line 2-2, and additionally showing a fill materialfilling the cube corner cavities and bonding the cover layer to the bodylayer;

FIG. 3 depicts a process for fabricating cube corner cavity-basedretroreflective sheeting; and

FIGS. 4A-C demonstrate a self-replicating phenomenon observed with sometypes of fill materials.

In the drawings, the same reference symbol is used for convenience toindicate elements that are the same or that perform the same or asimilar function.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

In FIG. 1, a portion of a retroreflective sheeting 10 is shown enlarged.Sheeting 10 comprises a body layer 12 having a structured surface 14,and a transparent cover layer 16. Structured surface 14 includesrecessed faces 18 and top surfaces 20, the recessed faces 18 formingcube corner cavities 22. The recessed faces 18 are shown shaded forvisual effect. In a preferred construction, a vapor-coated film ofreflective material such as aluminum, silver, or the like is exposed onthe recessed faces but masked on top surfaces 20, whether by the absenceof such reflective material or the presence of a masking material on thetop surfaces. Alternatively, the film of reflective material can beexposed on both faces 18 and surfaces 20, but the surfaces 20 arephysically roughened to impart a diffuse reflectivity to the film. Instill another alternative, top surfaces 20 can be eliminated by allowingthe recessed faces to converge or intersect along sharp edges.

FIG. 2 shows a sectional view of a portion of the sheeting 10,additionally showing a discontinuous film 24 of reflective material onthe recessed faces 18, and a fill material 26 that fills cube cornercavities 22. Fill material 26 is preferably sufficiently transparent toallow light rays to propagate through it with minimal degradation ofretroreflective efficiency. In contrast to known constructions, fillmaterial 26 forms a strong bond not only with transparent cover layer 16but also with film 24 and with any exposed portions of body layer 12.Thus, fill material 26 is preferably coextensive with the structuredsurface 14 and, for ease of construction, forms a substantiallycontinuous layer that covers both the recessed faces and the topsurfaces of the structured surface. In an alternative embodiment thefill material can be coextensive with structured surface 14 butdiscontinuous, encapsulated by a network of bonds directly between coverlayer 16 and top surfaces 20. This may be advantageous where, forexample, a direct bond between the cover layer and the body layer can bemade stronger than one in which the fill material and/or the reflectivematerial are interposed. However, embodiments having a continuous fillmaterial layer are preferred in part because the construction process ismore robust by avoiding the stringent requirement of having to apply aprecise amount of fill material to the structured surface—just enough tosubstantially fill the cavities, but not so much that the fill materialcovers the upper portions of the structured surface in a way thatinterferes with the network of bonds between the cover layer and upperportions of the body layer. The continuous fill material layerembodiments also allow the fill material to flow from one cube cornercavity to another before the fill material is solidified bycross-linking. Finally, in constructions where the fill materialfunctions as a bonding agent between the body layer and the cover layer,a continuous fill material layer improves bond strength by increasingthe surface area of the bond.

Market demands often require sheetings of various colors for differentapplications, or sheetings that have symbols or other indicia. Thesedistinctive visual effects can be realized by adding colorants, dyes, orthe like to cover layer 16 as is known. Manufacturing, inventory, andstorage costs become a consideration when a variety of differentsheetings each having different cover layers 16 must all be manufacturedand stored in sufficient quantity so that each type will be availableupon receipt of an order. It has been found that cube cornercavity-based sheeting that uses a PSA as the fill material hassurprising versatility that can be used to reduce these costs. Inparticular, the body layer, reflective film, and the fill material canall be prepared, but then instead of applying the transparent coverlayer 16, a standard release liner is laminated to the fill material.The release liner need only protect the fill material (the PSA) fromcontamination during storage until a particular type of sheeting iscalled for. At that time, the release liner is stripped and theappropriate cover layer is applied to the sheeting in a simplelamination process.

Heat-activated adhesives can be used as the fill material with similarbeneficial results. Examples of such adhesives are Nucrel brand ethyleneacid copolymer resins, sold by E. I. du Pont de Nemours and Company. Anadvantage of heat-activated adhesives is that the release liner can insome cases be eliminated from the construction. The intermediatesheeting, without the cover layer and without a release liner, can bestored in a roll under standard storage conditions without adhering toitself. A disadvantage is that the sheeting must be heated to activatethe adhesive properties when the cover layer is applied.

A PSA or heat-activated adhesive can also be used as the fill materialin applications where no cover layer is required. For example, it may bedesirable to apply the front side of the sheeting to a transparentsubstrate such as window glass in a vehicle or a building. The sheetingthus retroreflects light incident from the opposite side of the windowglass. For such applications a sheeting with a transparent PSA fillmaterial and a release liner is particularly well suited.

For most other applications, sheeting 10 preferably includes anotherthin adhesive layer 28 on the back side of body layer 12 so the sheetingcan be applied to a substrate of interest. Where layer 28 is a PSA,another release liner 28 a is also included. Layer 28 need not betransparent and thus it can comprise a wider variety of PSAs than layer26. However, if the same composition is used for layer 26 and 28,manufacturing inventory can be reduced.

Radiation-curable materials that are not PSAs or PSA precursors can alsobe used to advantage. See, e.g., Examples 1-4 below. Such materialsshould have sufficient clarity to promote good retroreflectivity, haverelatively low viscosity during application to the structured surface,and also have a sufficiently low shrinkage so that it maintains intimatecontact with the structured surface after curing.

Transparent PSA fill materials, particularly radiation curable fillmaterials disclosed herein, tend to be relatively expensive compared tofill materials disclosed in the prior art. Therefore, to keep productioncosts down it is advantageous when using the disclosed fill materials touse relatively inexpensive thermoplastic materials for body layer andcover layer compositions. However, other materials such asradiation-curable materials are also contemplated.

Product flexibility is often desirable in sheeting applications. At thesame time, the sheeting is expected to have a robust constructioncapable of withstanding various types of physical abuse. Theseconflicting requirements can be satisfied to some extent in the presentconstructions by the use of a fill material layer that has a relativelylow elastic modulus, less than about 50,000 psi (345 MPa), andpreferably less than about 25,000 psi (172 MPa), to provide flexibility.The fill material layer is sandwiched between and protected by the bodylayer and cover layer.

FIG. 3 (not drawn to scale) depicts a process for making cube cornercavity-based sheeting with the preferred fill materials. Not shown inthe figure, body layer 12 described above is provided the structuredsurface 14 by embossing, or by other processes used to make conventionalrear-surface body layers. Also not shown, a film of reflective material24 is then applied either discontinuously as shown in FIG. 2, orcontinuously on both the recessed faces and the top surfaces 20. It isnot necessary that the structured surface 14 have top surfaces 20,although such surfaces are useful for controlling daytime appearanceand, in some instances, for improved bonding. In the absence of topsurfaces 20, recessed faces 18 of adjacent cube corner elementsintersect to form sharp edges.

Film 24 can comprise metals such as aluminum, silver, nickel, tin,copper, or gold, or combinations thereof, or can comprise non-metalssuch as a multilayer dielectric stack. Such films can be applied byknown physical or chemical deposition techniques, such as vacuumevaporation, sputtering, chemical vapor deposition (“CVD”) orplasma-enhanced CVD, electroless deposition, and the like, dependingupon the type of film desired. A given film can include multiple layers,including layers that promote adhesion to the body layer, barrierlayers, and protective overcoat layers. A suitable film forpolycarbonate-based body layers comprises about 1 nm thick titaniumdioxide layer formed by sputtering titanium onto the body layer,followed by a 100 nm thick layer of evaporated aluminum. The titaniumdioxide layer acts both as an adhesion promoter and a barrier layer tocounteract pinholes typically present in the aluminum vapor coat.

The body layer 12 so prepared is then sent through a fill materialapplication station 30. Generally, the more easily the fill materialfills the cavities, the faster (and hence cheaper) the process can berun. Preferred fill materials have properties that permit rapid fillingof the cube corner cavities. The fill material should adhere to thereflective film-covered recessed cube corner cavity faces withoutdamaging the reflective film or other parts of the structured surface.

At station 30, a flowable fill material composition 32 is applied to thestructured surface 14 ahead of a knife coater 34 whose position relativeto a base plate 36 is adjusted to form a layer of composition 32 on thestructured surface. If desired, vacuum assistance or inert gas purgingcan be used at the point of filling to further facilitate the operation.Some compositions 32, discussed in more detail below, have a relativelylow viscosity at station 30 to permit rapid filling of the closed cubecorner cavities. In contrast to prior art thermoplastic fill materialssuch compositions can exhibit these low viscosities at relatively lowprocess temperatures, well below the glass transition temperature oftypical body layer materials. For some fill materials, processtemperatures at or around ambient room temperature are achievable.

Composition 32 is suitable for forming a fill material that bonds wellto all other parts of the sheeting that it contacts, including thereflective film 24, exposed portions of the body layer 12, and any coverlayer (with the exception of a release liner that may be used as atemporary cover layer).

As an aside, where a discontinuous reflective film 24 is used, somecombinations of fill material and body layer material can be used toproduce a covalent bond therebetween for added robustness anddurability. The covalent bond between the fill material and body layercan be formed during exposure to radiation. For example, for a fillmaterial composition comprised of 25 wt. % tetrahydofurfuralacrylate(THF acrylate), 50 wt. % Ebecryl 8402 (available from Radcure), and 25wt. % neopentylglycoldiacrlylate, a suitable body layer can comprise anethylenepropylenedienemonomer (EPDM) based material, such asacrylic-EPDM-styrene (AES) polymers sold under the tradename Centrex(available from Bayer), Luran (available from BASF), or other polymersthat crosslink upon exposure to radiation. Such fill material reactswith these body layer materials at the top surfaces 20 upon exposure tocrosslinking radiation to produce covalent bonds along the top surfaces.

Turning again to FIG. 3, after composition 32 is applied to thestructured surface at fill material application station 30, the filledbody layer is conveyed to a cover layer laminating station 38. For somefill materials, however, it may be desirable to apply radiation, such aswith a source of radiation 40, after station 30 but before station 38.For example, some compositions 32 are composed of highly monomerizedsyrups which can chemically attack certain reflective films 24 ormigrate through pinholes in the reflective film to attack the underlyingbody layer material. In such cases, it is preferable to polymerize andcrosslink the compositions in situ shortly after application to the bodylayer so that damage to the sheeting can be minimized. However, forother compositions it is desirable that the composition 32 remainflowable at least up to the laminating station 38.

At station 38, filled body layer 12 is conveyed between pressure rollers42 a,42 b rotating oppositely as shown. A cover layer 44, unwound from aroll 46, passes through the nip between the rollers and is laminated tobody layer 12. The fill material, though preferably still not fullycross-linked, exhibits sufficient adhesion to hold cover layer 44 inplace. The stippled appearance of composition 32 in FIGS. 3 and 4A-Cindicates that it is not fully crosslinked and exhibits cold flow.

In an alternative embodiment, the composition 32 can be applied to thestructured surface 14 by first being applied to the underside of thecover layer 44. Then, filling of the cube corner cavities and laminationof the cover layer can take place simultaneously at the laminationstation 38.

Cover layer 44 can comprise a transparent cover layer such as layer 16,to be used in the final sheeting product, or it can comprise a temporarylayer such as a release liner. In either event another source ofradiation 48 can be used to crosslink composition 32 to increase itsshear and adhesive strength. Such crosslinked fill material is labeled32 a and depicted as cross-hatched rather than stippled. The fillmaterial 32 a does not exhibit significant cold-flow. In a simpleconstruction method, cover layer 44 is a transparent cover layer such aslayer 16. Source 48 is sufficiently intense, and layer 44 has a lowenough absorption for at least some UV wavelengths, or for e-beamradiation, so that crosslinking can be effected through the cover layeras shown. The crosslinked composition 32 a aggressively bonds toreflective film 24, body layer 12, and cover layer 44. In an alternativeconstruction method, the cover layer 44 has a higher UV absorption tobetter protect the remainder of the sheeting from degradation due tosunlight. Body layer 12 and reflective film 24 are then composed ofmaterials having a lower absorption at the relevant UV wavelengths, andsource 48 is disposed underneath instead of above the sheeting so as toexpose the composition 32 to crosslinking radiation through body layer12 and film 24. Silver used as reflective film 24 can be madesufficiently thin to allow adequate transmission in a UV spectral bandlocated at about 360 nm. Alternatively, multilayer dielectric films canbe easily tailored to have a high specular reflectance at the designwavelength and a transmission band in the UV.

In still another method, cover layer 44 is a release liner. The releaseliner is of conventional design, for example silicone-coated paper. Therelease liner may or may not be transparent at the design wavelength forthe sheeting, and it may or may not have a low absorption of UVradiation. If absorption in the UV is low enough, source 48 cancrosslink composition 32 through layer 44 as shown in FIG. 3. Otherwise,the release liner can be removed just prior to crosslinking, or source48 can be positioned to crosslink composition 32 from below the sheetingas discussed above.

Sources 40, 48 can be adapted to emit UV light or other forms ofradiation capable of carrying out the prescribed functions, for example,infrared radiation or electron beam radiation.

Still other methods contemplated herein delay application ofcross-linking radiation from source 48 to take advantage of the coldflow characteristics of composition 32. For example, a temporarysheeting comprising body layer 12, cover layer 44, and fill material 32can be manufactured as shown in FIG. 3 except that source 48 iseliminated and the temporary sheeting is wound up in a roll and placedin storage. During storage, imperfections in the fill material layertend to disappear due to flow of the uncured composition 32 from forcessuch as surface tension or other forces incident to the storageenvironment. After a sufficient time has elapsed, the temporary sheetingcan then be further processed by rapidly passing it near source 48 toexpose composition 32 to radiation sufficient to produce crosslinkedcomposition 32 a, preferably a PSA. In some cases a relatively shortdelay may suffice, so that the temporary sheeting need not be wound upand stored.

Overall processing speed for the sheeting can be enhanced by using thedelayed curing procedure just described. This is because the fillingoperation at station 30 can be accelerated beyond the speed at whichsubstantially complete filling of cavities 22 is assured. Because thefill material is radiation curable, it remains in its uncured flowablestate substantially indefinitely—whether for seconds, minutes, hours, ordays—until crosslinking by exposure to radiation is needed. Theradiation-curable materials can thus be used to improve the process ofmaking cube corner cavity-based retroreflective sheeting, making theprocess faster and more robust since the degree of care needed to ensurecomplete filling of the cube corner cavities is not required.

FIGS. 4A-C depict the sheeting 10 at different times after applying fillmaterial to the structured surface 14 and after laminating cover layer44 thereto, but before crosslinking the fill material. The productionline speed of the body layer 12 has been increased to the point wherecomplete filling of the cavities has not occurred, leaving a void 50 atthe apex of each cavity. The cold flow properties of the composition 32advantageously allow the fill material to advance into the cavitieswithout the application of external forces and, typically, at standardprocess or storage temperatures, typically about 10 to about 40 degreesC. This behavior is referred to as self replication. As composition 32advances into the cavity, voids 50 shrink (FIG. 4B), eventuallydisappearing altogether (FIG. 4C). Self replication via shrinkage of thevoids is presumed to occur as gas in the voids diffuses into the fillmaterial. The rate at which self replication occurs is dependent on thetype of composition 32 used, the cover layer 44 properties (if a coverlayer is used), the size of the cube corner cavities and the initialsize of gaps 50, and on environmental factors such as temperature.

Illustrative Fill Materials

Suitable fill materials include viscoelastic polymers which can attainthe requisite transparency. Preferred materials exhibit cold flow atroom temperature which allows the fill material to flow into thecavities of the body layer and also allows any entrapped gas to diffuseout, thus maintaining the optical performance of the sheeting. It isdesirable further that the fill material exhibit little or no shrinkageupon curing such that it maintains intimate contact with the reflectiverecessed faces of the structured surface.

A preferred class of materials includes acrylic polymers which may bepressure-sensitive adhesives at room temperature, or heat-activatedadhesives which are substantially non-tacky at room temperature butbecome tacky at higher temperatures. The preferred acrylic polymers andcopolymers are formed from acrylic or methacrylic acid esters ofnon-tertiary alkyl alcohols. The acrylic and methacrylic esterstypically have a glass transition temperature below about 0° C. Examplesof such monomers include n-butyl acrylate, isooctyl acrylate,2-ethylhexyl acrylate, isononyl acrylate, isodecyl acrylate, decylacrylate, lauryl acrylate, hexyl acrylate, and octadecyl acrylate, orcombinations thereof and the like. Such monomeric acrylic or methacrylicesters are known and many are commercially available.

The acrylic polymers typically include a copolymerizable monomer havinga glass transition temperature above 0° C. to enhance shear strength.Suitable copolymerizable monomers include acrylic acid, methacrylicacid, itaconic acid, n-vinyl pyrrolidine, n-vinyl caprolactam,substituted acrylamides such as N,N dimethylacrylamide,N-vinyl-2-pyrrolidone, N-vinyl caprolactam, acrylonitrile, isobornylacrylate, tetrahydrofurfuryl acrylate, glycidyl acrylate, 2phenoxyethylacrylate, benzylacrylate, acrylonitrile andmethacrylonitrile or combinations thereof.

Typically, the amounts of acrylate monomer to copolymerizable monomercan vary from 100 to about 30 parts of acrylate and correspondingly, 0to 70 parts of copolymerizable monomer. The specific amounts of monomersare selected for the desired end use properties.

The acrylic polymers may be prepared by emulsion polymerization, bulkpolymerization, solvent polymerization, and the like, using appropriatepolymerization initiators. Suitable pressure-sensitive adhesives for theinvention are described in, for example, U.S. Pat. No. 5,637,646(Ellis), U.S. Pat. No. 4,181,752 (Martens et al.), and Re. 24,906(Ulrich), all incorporated herein by reference.

Various other materials may be added to tailor the characteristics ofthe polymer for the end use. Such materials include colorants,fluorescent dyes or pigments, chain transfer agents, plasticizers,tackifiers, antioxidants, stabilizers, cross-linking agents, andsolvent.

The fill material is preferably crosslinked to provide higher shearstrength.

In order to maintain optical performance, it is preferred that anyentrapped gas or voids be allowed to escape or collapse before the fillmaterial is crosslinked. Suitable cross-linking agents include thosethat are free radically copolymerizable with the acrylate monomers, andmay be activated by radiation such as ultraviolet light. Additionally,crosslinking may be effected in the absence of cross-linking agents bymeans such as electron beam.

When the fill material is applied to the sheeting in substantiallypolymeric form, e.g. as a hot melt coating, entrapped gas is allowed todiffuse, presumably into the fill material, before crosslinking.Examples of crosslinking agents suitable for this application includefree-radically copolymerizable crosslinking agents such as, for example,4-acryloxybenzophenone, para-acryloxyethoxybenophenone, andpara-N-(methacryloxyethyl)-carbamoylethoxybenophenone. Copolymerizablechemical cross linking agents are typically included in the amount ofabout 0% to about 2%, and preferred in the amount of about 0.025% toabout 0.5%, based on the total weight of monomer(s). Other usefulcopolymerizable crosslinking agents are described in U.S. Pat. No.4,737,559 (Kellen et al.). The crosslinking is effected by ultravioletlight.

Alternatively, a fill material composition may be polymerized in situ inthe cavities of the sheeting by coating a monomeric or oligomericcomposition onto the sheeting and polymerizing the compositions withheat or radiation. In this case, the composition has a viscosity that issufficiently low prior to polymerization that any gas such as airdiffuse out of the composition quickly prior, and the composition flowsrapidly and easily to fill the cavities of the sheeting. Suitablecrosslinking agents include those mentioned above as well as materialsthat crosslink during the polymerization process. Examples of this typeof crosslinking agent include multi-functional acrylates such as1,6hexanedioldiacrylate and trimethylolpropanetriacrylate andsubstituted triazines described in U.S. Pat. No. 4,330,590 (Vesley etal.) and U.S. PAT. NO. 4,329,384 (Vesley at al.). These crosslinkingagents may be used in amounts of from about 0.0001% to about 0.005%based on the weight of the monomers.

The fill material may be applied to the sheeting by any suitable method.For example, the polymers can be dispersed in a solvent or an emulsion,coated onto the sheeting, and drying off the solvent or water to leavethe polymer in the cavities of the sheeting. The polymers may be hotmelt coated onto the sheeting using known equipment such as extrusioncoaters, rotary rod die coaters, and the like. The polymers may also beformed in the cavities of the sheeting as described above. Solvent-freeprocesses are preferred because they eliminate environmental concernsassociated with solvents and avoid formation of bubbles which may occurduring drying of a solvent-containing composition.

Body Layer Materials and Cover Layer Materials

The body layer for retroreflective sheeting as described herein can bemanufactured as a unitary material, e.g. by embossing a preformed sheetwith an array of cube corner elements as described above or by casting afluid material into a mold. Alternatively, the body layer can bemanufactured as a layered product by casting a layer defining thestructured surface against a preformed flat film analogous to theteachings of PCT Publication No. WO 95/11464 (Benson, Jr. et al.) andU.S. Pat. No. 3,684,348 (Rowland), or by laminating a preformed film toa preformed layer having cube corner cavities. Useful body layermaterials are those that are dimensionally stable, durable, weatherable,and readily formable into the desired configuration. Examples includeacrylics such as Plexiglas brand resin from Rohm and Haas, thermosetacrylates and epoxy acrylates, preferably radiation cured;polycarbonates; polyolefins; polyethylene-based ionomers (marketed underthe name ‘SURLYN’); polyesters; cellulose acetate butyrates; andpolystyrenes. Generally any material that is formable, typically underheat and pressure, can be used. The sheeting can also include colorants,dyes, UV absorbers, or other additives as desired.

Suitable transparent cover layer materials can also be of single ormultilayer construction and can comprise materials that are opticallytransparent at least at a design wavelength and that are durable andweatherable. Thermoplastic or thermoset polymers, or combinationsthereof, are generally acceptable. Acrylics, vinyl chloride, urethanes,ethylene acrylic acid (EAA) copolymers, polyesters, and fluoropolymersincluding polyvinylidene fluoride are preferred for weatherability.Corrosion inhibitors, UV stabilizers (absorbers), colorants includingfluorescent dyes and pigments, abrasion resistant fillers, solventresistant fillers, and the like can be included to provide desiredoptical or mechanical properties. The cover layer can have graphics,symbols, or other indicia so that the sheeting formed by the combinationof the body layer and cover layer conveys useful information.

As discussed above, many thermoplastic polymers such as EAA, polyvinylchloride, polystyrene, polyethylene-based ionomers,polymethylmethacrylate, polyester, and polycarbonate are as a wholerelatively inexpensive and for that reason, to help offset the higherexpense of typical radiation cured fill materials disclosed herein, theyare desirable for use in the body layer and cover layer. The body layeralso preferably has an elastic modulus greater than that of the fillmaterial in order to maintain cube stability during deformation. Thebody layer elastic modulus is preferably greater than about 100,000 psi(690 MPa), measured in accordance with ASTM D882-97 “Standard TestMethod for Tensile Properties of Thin Plastic Sheeting”.

EXAMPLES 1-4

Four body layers were embossed with a mold to impart a structuredsurface similar to that shown in FIG. 1. The mold had a structuredsurface consisting of three sets of flat-bottomed grooves, and was thenegative replica of a prior mold whose upper portions had been grounddown flat with an abrasive. The embossed body layers were made ofpolycarbonate. The body layers for Examples 1 and 2 had a thickness ofabout 43 mils (1.1 mm) and included sufficient TiO₂ filler to make themopaque with a diffuse white surface appearance. Those for Examples 3 and4 had a thickness of about 18 mils (0.46 mm) and included instead a reddye to give a diffuse red surface appearance. The structured surface ofeach body layer consisted essentially of three intersecting sets ofparallel ridges. Two of the sets, referred to as “secondary” ridge sets,had uniform ridge spacings of about 16 mils (408 μm) and intersectedeach other at an included angle of about 70 degrees. The other set ofparallel ridges, referred to as the “primary” ridge set, had a uniformspacing of about 14 mils (356 μm) and intersected each of the secondaryridge sets at an included angle of about 55 degrees. This produced cubecorner cavity matched pairs canted at an angle of about 9.18 degrees.All of the ridges had substantially flat top surfaces whose transversedimension was about 3.5 mils (89 μm) for the primary grooves and about2.2 mils (56 μm) for the secondary grooves. The top surfaces were allnon-smooth as a result of the abrasive action on the original molddiscussed above, transferred to the body layers via the replicationsteps. The cube corner elements had a cube depth below the top surfacesof about 5.17 mils (131 μm). A silver film was vacuum deposited onto thestructured surface of each sample to a thickness sufficient to renderthe film opaque yet highly reflective. For Examples 2 and 4, the portionof the silver film disposed on the top surfaces was removed by lightlysanding with an abrasive. The silver film for Examples 1 and 3 was leftundisturbed and continuous.

A radiation-curable composition was prepared by combining (by weight)74% Ebecryl 270 (a urethane acrylate available from Radcure), 25%Photomer 4127 (propoxylated neopently glycol diacrylate available fromHenkel), and 1% Daracure 1173 (a photoinitiator available fromCiba-Geigy). This composition was then flow coated on the structuredsurface of all samples at room temperature to a thickness sufficient tofill the cube corner cavities and cover the top surfaces. Thecomposition was flowable and had a viscosity of about 2000 centipoise (2Pa·s) during filling. The samples were degassed at room temperature in asmall vacuum chamber. Next, when no bubbles remained in the composition,the samples were removed from the chamber and covered with a 7 mil (178μm) thick sheet of photo-grade PET sheeting to eliminate oxygen duringsubsequent curing. A heavy quartz plate having good transparency in theUV was placed on the PET sheeting and curing was then performed throughthe quartz plate and PET sheeting with UV light from a mercury lamp forabout two minutes. The fill material composition had a sufficiently lowshrinkage so that it hardened and bonded to the vapor-coated body layer.The composition did not bond to the PET sheeting, which was thenremoved. The cured composition was substantially clear and smooth butnot permanently tacky. The sheetings so constructed all exhibitedretroreflectivity. The coefficient of retroreflection was measured at a−4 degree entrance angle, 0 degree orientation angle, and at both 0.2and 0.5 degree observation angles, and have not been adjusted to takeinto account the proportion of the structured surface actually occupiedby the cube corner elements: Body layer Silver film on Retro. Coeff.(cd/lx/m²) Sample No. color top surfaces @ 0.2° @ 0.5° 1 White Yes 58 262 White No 46 22 3 Red Yes 28 14.6 4 Red No 22 17

These measurements demonstrate that the silver film imparts a highspecular reflectivity to the recessed faces. Samples 2 and 4, with thesilver film exposed selectively on the recessed faces, exhibitednoticeable daytime color (white or red) as a result of the exposed bodylayer at the top surfaces.

EXAMPLES 5-16

Twelve body layers made of polystyrene (type 498 Styron brand, availablefrom Dow Chemical Co., Midland, Mich.) were embossed with a mold toimpart a structured surface consisting essentially of three intersectingsets of parallel ridges. Six of the body layers (Examples 5-10) werenatural, clear polystyrene and the remaining six (Examples 11-16) usedpolystyrene compounded with a titanium dioxide concentrate to impartdiffuse whiteness. The body layers were each about 9 mils (230 μm)thick. Two of the three sets of parallel ridges, referred to as“secondary” ridge sets, had uniform ridge spacings of about 5.74 mils(146 μm) and intersected each other at an included angle of about 70degrees. The other set of parallel ridges, referred to as the “primary”ridge set, had a uniform ridge spacing of about 5 mils (127 μm) andintersected each of the secondary ridge sets at an included angle ofabout 55 degrees. This produced cube corner cavity matched pairs havinga cavity depth of about 2.5 mils (64 μm) and an angle of cant of about9.18 degrees. The ridges in each of the three ridge sets did not havetop surfaces as defined herein but rather terminated at sharp upperportions whose transverse dimension was less than 0.0001 inches (2.5μm). A continuous aluminum film about 100 nm thick was vacuum depositedonto the entire structured surface.

A transparent hot melt PSA composition was prepared according to theprocedure of Example 1 of U.S. Pat. No. 5,753,768 (Ellis) except asindicated below. A 200-gallon (757 liter) stainless steel batch reactorwas charged with: 441.3 kilograms of isooctyl acrylate (“IOA”); 54.4 kgof acrylic acid (“AA”); 0.0017 parts of Vazo™ 52(2,2′-azobis(2,4-dimethylpentanenitrile)) per 100 parts of IOA and AA(“pph”); 0.0084 pph isooctylthioglycoate; 0.5 pph of a 25 weight %solids mixture of 4-acryloxy benzophenone in IOA; and 0.1 pph ofIrganox™ 1010 thermal stabilizer/antioxidant(tetrakis(methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate))methane),manufactured by Ciba-Geigy Corporation. The composition was reacted inthe first adiabatic reaction cycle with the starting reactiontemperature of about 60 degrees C. It reached a solids content of about50% from the reaction.

After cooling the composition to about 55 degrees C., a mixture of 0.004pph of Vazo™ 52, 0.004 pph of Vazo™ 88(2,2′-azobis(cyclohexanecarbonitrile)), 0.0004 pph of di-t-butylperoxide, 0.004 pph of t-butylperbenzoate, and 0.04125 pphisooctylthioglycoate, 0.5 pph of a 25 weight % solids mixture of4-acryloxy benzophenone in IOA, and 4.54 kilograms of IOA was mixed intothe reaction mixture.

The composition was then heated to 60 degrees C., held untilpolymerization began, and then reacted adiabatically. After the secondadiabatic reaction cycle was completed, the resulting polymercomposition had a 93% solids content.

The batch of adhesive thus produced was then vacuum stripped to providea pressure-sensitive adhesive polymer having an inherent viscosity of0.44 dl/g and 0.1% or less residuals. The inherent viscosity wasmeasured by conventional methods using a Cannon-Fenske #50 viscometer ina water bath controlled at 25° C., using the flow time of 10 ml of apolymer solution (0.2 g per deciliter polymer in ethyl acetate). Thetest procedure followed and the apparatus used are described in Textbookof Polymer Science, F. W. Billmeyer, Wiley-Interscience (Second Edition,1971), pp. 84-85.

The prepared PSA composition was substantially completely polymerizedand was permanently tacky at room temperature. The composition exhibitedcold flow, yet strips of it could be cut from a bulk slab with a knife.

The prepared PSA composition was then fed via a Haake 18 mm twin screwextruder into a 5 inch wide rotary rod die coater which applied thecomposition to the structured surface of the twelve body layers by hotmelt coating. A rubber coated backup roll with a temperature varyingbetween about 90 and 180 degrees F. (32 and 82 degrees C.) was usedduring this application step. The composition temperature as applied wasdetermined by the extruder and die coater temperatures, ranging fromabout 325 to 375 degrees F (163 to 191 degrees C.). Line speed of thebody layer during fill material application was between about 10 and 20ft/min (51 to 102 mm/sec). The applied composition coating wascontinuous and had a thickness of about 1-1.5 mils (25 to 38 μm)measured from the uppermost portions of the ridges on the structuredsurface. In all cases, voids between the composition and the reflectivefilm were observed at the cavity apices indicating poor replication—thecomposition had not fully filled the cube corner cavities (see FIG. 4A).The voids appeared to occupy approximately 10-40% of the cube cornercavity volume. The observed retroreflectivity was poor. For Examples 5-7and 11-13 the PSA composition was left as coated and for the othersamples the composition was crosslinked by exposure to a dosage of about440 mJ/cm² of UV light as measured with an EIT UVIMAP, NIST unitscalibrated in the UVA spectral range. The crosslinked composition formeda PSA with high shear and cohesive strength with little or no cold flow.Three different transparent cover layers, each about 2 mils (50 μm)thick were then laminated to the PSA composition layer of the samples atambient room temperature—an extruded impact-modifiedpolymethylmethacrylate (PMMA) film was laminated to Examples 5, 8, 11,and 14, a plastisol-coated plasticized polyvinyl chloride (PVC) film waslaminated to Examples 6, 9, 12, and 15, and an extruded polyethyleneco-acrylic acid (EAA) film was laminated to Examples 7, 10, 13, and 16.

The samples were wound up in rolls and maintained at ambient roomtemperature. Within several hours after lamination, the non-crosslinkedsamples (Examples 5-7 and 11-13) began to show significantly improvedretroreflective performance indicative of the fill material flowing tomore completely fill the cube corner cavities. After about 24 to 72hours the fill material layer for those samples was substantially freeof any voids. Apparently, the matter filling the voids diffused out ofthe sheeting during this time. The samples with the acrylic (PMMA) coverlayer appeared to allow the most rapid rate of self replication; thosewith the EAA cover layer were somewhat slower; and those with the vinyl(PVC) cover layer yielded the slowest self replication rates of thesamples tested. The crosslinked samples, however, showed no visibleimprovement in replication fidelity of the fill material layer evenafter several months of storage at ambient room temperature.

Samples similar to Examples 5, 7-11, and 13-16 were made except that thebody layer comprised polycarbonate (Makrolon brand, type 2407, availablefrom Bayer) instead of polystyrene. The results followed the samepattern as the polystyrene samples. Also, the effect of the use ornon-use of reduced gas pressure at the fill material application stationwas investigated; no effect was noted.

The non-crosslinked samples, after complete replication of the fillmaterial into the cube corner cavities of the structured surface, werethus ready for subsequent exposure to radiation sufficient to crosslinkthe fill material composition to increase its shear strength bysolidification, while still maintaining its PSA characteristics. Thecoefficient of retroreflection was measured for all the non-crosslinkedsamples for −4 degree entrance angle and 0.2 degree observation angle,with values ranging from about 589 to about 982 cd/lx/m².

EXAMPLE 17

A roll of body layer retroreflective sheeting substantially the same asthose of Examples 5-16 was prepared. The same structured surfacegeometry and reflective aluminum film was used.

A pressure-sensitive adhesive resin composition was prepared by mixing75 parts of isooctyl acrylate and 25 parts of N-vinylcaprolactam toyield about 2000 grams. Then 0.05 pph of a photoinitiator(2,4,6-trimethylbenzoyldiphenylphosphine available as Lucirin™ TPO fromBASF Corp.) was added. Nitrogen was bubbled through the composition andthe composition was exposed to Sylvania black lights to partiallypolymerize it to a viscosity of about 1700 centipoise (1.7 Pa·s). Thecomposition viscosity was measured using a model LVF BrookfieldViscometer equipped with a number 4 spindle at 60 rpm at roomtemperature. In the partial polymerization process, the temperature ofthe composition increased from 23° C. to about 38° C. The compositionwas then sparged with air and cooled to room temperature. An additional0.15 pph of photoinitiator (Lucirin™ TPO) and 0.15 pph1,6-hexanedioldiacrylate were added to it. The partially polymerizedcomposition, still substantially monomeric (<10% polymerized), was knifecoated onto the body layer sheeting to a thickness of about 2 mils (50μm) measured from the uppermost portions of the ridges on the structuredsurface, and then exposed to ultraviolet radiation in a nitrogenatmosphere to cure (polymerize and crosslink) the adhesive in situ. Theultraviolet radiation was provided by ultraviolet black lamps havingmost of the light emission between 300 and 400 nanometers, and a peakemission at about 350 nanometers. The light intensity averaged about 4.9mW/cm², and the total energy was about 498 mJ/cm². The UV light wasmeasured with an EIT UVMAP in NIST units. Due to the low viscosity andthe wetting characteristics of the composition to the aluminum vaporcoat, no voids were seen in the fill material within seconds of coatingat the coating speed of 20 feet per minute. After curing, the adhesivewas covered with a silicone-coated polypropylene release liner.

Afterwards the release liner was removed and a transparent cover layerof PMMA film similar to those used in Examples 5, 8, 11, and 14 about 2mils (50 μm) thick, was laminated to the pressure-sensitive adhesive onthe body layer to produce a sheeting construction. This Example 17exhibited a good coefficient of retroreflectivity: the average value ofmeasurements taken at 0 and 90 degree orientation angles, at a −4 degreeentrance angle and 0.2 degree observation angle, was 1002 cd/lx/m².

Glossary of Selected Terms

-   “Adhesive” means a substance suitable for bonding two substrates    together by surface attachment.-   The “body layer” of a retroreflective sheet or article that uses a    structured surface for retroreflection is the layer (or layers)    possessing the structured surface and chiefly responsible for    maintaining the integrity of such structured surface.-   “Cold flow” refers to the ability of a material to flow under its    own weight at ambient room temperature, about 20 degrees C.-   “Cube corner cavity” means a cavity bounded at least in part by    three faces arranged as a cube corner element.-   “Cube corner element” means a set of three faces that cooperate to    retroreflect light or to otherwise direct light to a desired    location. “Cube corner element” also includes a set of three faces    that itself does not retroreflect light or otherwise direct light to    a desired location, but that if copied (in either a positive or    negative sense) in a suitable substrate forms a set of three faces    that does retroreflect light or otherwise direct light to a desired    location.-   “Cube corner pyramid” means a mass of material having at least three    side faces arranged as a cube corner element.-   “Cube height” or “cube depth” means, with respect to a cube corner    element formed on or formable on a substrate, the maximum separation    along an axis perpendicular to the substrate between portions of the    cube corner element.-   “Diffusely reflective”, “diffuse reflectivity”, and cognates thereof    mean the property of reflecting a collimated incident light beam    into a plurality of reflected light beams. Surfaces that are    diffusely reflective also have a low specular reflectivity.-   “Flowable” refers to the ability of a material to flow under its own    weight at a given temperature.-   “Geometric structure” means a protrusion or cavity having a    plurality of faces.-   “Groove” means a cavity elongated along a groove axis and bounded at    least in part by two opposed groove side surfaces.-   “Groove side surface” means a surface or series of surfaces capable    of being formed by drawing one or more cutting tools across a    substrate in a substantially continuous linear motion. Such motion    includes fly-cutting techniques where the cutting tool has a rotary    motion as it advances along a substantially linear path.-   “Heat-activated adhesive” means a solid thermoplastic material that    melts upon heating and then sets to a firm bond upon cooling.-   “X% polymerized” means 100% minus the weight % of unreacted monomer    in a composition.-   “Pressure-sensitive adhesive” (abbreviated “PSA”) means a    permanently tacky material capable of adhering to surfaces upon    applying at least a slight amount of manual pressure.-   “Radiation curable” means the capacity of a composition to undergo    polymerization and/or crosslinking upon exposure to ultraviolet    radiation, visible radiation, electron beam radiation, or the like,    or combinations thereof, optionally with an appropriate catalyst or    initiator.-   “Retroreflective” means having the characteristic that obliquely    incident incoming light is reflected in a direction antiparallel to    the incident direction, or nearly so, such that an observer at or    near the source of light can detect the reflected light.-   “Structured” when used in connection with a surface means a surface    composed of a plurality of distinct faces arranged at various    orientations.-   “Symmetry axis” when used in connection with a cube corner element    refers to the axis that extends through the cube corner apex and    forms an equal angle with the three faces of the cube corner    element. It is also sometimes referred to as the optical axis of the    cube corner element.-   “Transparent”, with regard to a layer or substance for use in a    retroreflective sheeting, means able to transmit light of a desired    wavelength to a degree that does not prevent retroreflection.-   “Top surfaces” of a structured surface that also contains recessed    faces refers to surfaces that are distinct from the recessed faces    and that have a minimum width in plan view of at least about 0.0001    inches (2.5 μm).-   “Viscosity” means the internal resistance to flow exhibited by a    fluid at a given temperature. Low to moderate viscosities are    typically measured using a rotating spindle in contact with the    fluid and expressed in SI units of Pascal-seconds (Pa·s). The    viscosity of highly viscous polymers can be measured by dissolving    the polymer in a solvent and comparing the efflux times, as    described in Textbook of Polymer Science, F. W. Billmeyer,    Wiley-Interscience (Second Edition, 1971), pp. 84-85. A viscosity    measured by the latter approach is referred to as “inherent    viscosity” and expressed in units of inverse concentration, such as    deciliters per gram (dl/g).

All patents and patent applications referred to herein are incorporatedby reference. Although the present invention has been described withreference to preferred embodiments, workers skilled in the art willrecognize that changes can be made in form and detail without departingfrom the spirit and scope of the invention.

1-34. (canceled)
 35. A method of making a cube corner article,comprising: providing a body layer having a structured surface thatincludes recessed faces defining cube corner cavities; applying a filmof reflective material to the recessed faces; applying to the structuredsurface a radiation-curable composition suitable for bonding to the filmof reflective material; and exposing the composition to radiationsufficient to crosslink the composition after the composition has filledthe cube corner cavities. wherein the body layer has an elastic modulusthat is greater than about 690 MPa and the radiation-cured, cross-linkedcomposition has an elastic modulus that is less than about 690 MPa. 36.The method of claim 1, wherein the radiation-cured, cross-linkedcomposition has an elastic modulus that is less than about 345 MPa. 37.The method of claim 1, wherein the second applying step applies thecomposition at a thickness sufficient to form a composition layercovering the recessed faces.
 38. The method of claim 1, furthercomprising: providing a first cover layer; and laminating the firstcover layer to the article.
 39. The method of claim 4, wherein the firstcover layer has the flowable composition applied thereto, and the secondapplying step is carried out by the laminating step.
 40. The method ofclaim 4, wherein the first cover layer comprises a release liner thatdoes not bond to the composition.
 41. The method of claim 6, furthercomprising: removing the release liner; providing a second cover layersuitable for bonding to the composition; and laminating the second coverlayer to the composition.
 42. The method of claim 1, wherein the secondapplying step is carried out such that the flowable compositionincompletely fills the cube corner cavities.
 43. The method of claim 8,further comprising: providing a cover layer; and laminating the coverlayer to the article before the flowable composition has filled the cubecorner cavities.
 44. The method of claim 1, wherein the flowablecomposition is at least 95% polymerized during the second applying step.45. The method of claim 1, wherein the structured surface furtherincludes a top surface separating the cube corner cavities.